1. Field of the Invention
The present invention relates to an optical pickup device for writing and reading information in conjunction with a magnetooptical disk or other recording mediums.
2. Description of the Related Art
An example of the optical pickup device is described below.
The device comprises a fixed optical system and a movable optical system. The fixed optical system is composed of a semiconductor laser source, a coupling lens, a polarization beam splitter and a quarter-wave plate. A laser beam emitted from the laser source is collimated by the coupling lens. The collimated beam passes through the beam splitter and the quarter-wave plate, respectively, and propagates out of the fixed system toward the movable system.
The movable system comprises a carriage, a deflection prism mounted on the carriage and an objective lens also mounted on the carriage. The system is movably disposed below an optical disk.
The laser beam output from the fixed system enters into the movable system in which the beam is deflected perpendicularly by the prism and converged by the lens to a point on the optical disk surface so that information data are written on the disk.
The beam is reflected and scattered from the irradiation point on the disk surface. A part of the reflection light enters back into the movable system. The reflection light propagates back through the lens and the prism, repectively, in the movable system and enters again into the fixed system. In the fixed system, the light is reflected by the beam splitter to be guided into an optical signal detection system which reads information written in the disk and detects a focus error signal or a track error signal from the reflection light.
However, the optical pickup device mentioned above involves problems as follows.
First, the optical axis of the laser beam transmitted from the fixed system to the movable system in which the beam is reflected by the prism to the objective lens is inclined by angle .theta. with respect to the optical axis of the lens due to inaccuracy of the prism shape, for example, in the event that the deflection surface of the prism is not accurately formed with an inclination angle of 45 degrees with respect to the bottom surface thereof or dislocation of the prism due to an assembly error.
Also, if the prism 8 is mounted in the state of inclining by angle .theta., the laser beam enters the objective lens with an inclination of angle 20 to the optical axis of the objective lens.
Such a misalignment of optical axes between the lens and the incident beam causes astigmatism and coma aberration so that the spot shape of the converge*d beam irradiated to the surface of the disk is deformed, which degrades the imaging state of the beam and lowers the reliability of signal detection, accordingly.
To avoid such a problem, it becomes necessary to raise the accuracy of the prism shape and the assembly location and attitude of the prism, which results in raise of the cost.
Besides, with respect to the separation type pickup device, that is, the device which is divided to two parts, i.e., the fixed optical system and the movable optical system, the device has to be arranged in such a way that the optical axis of the beam output from the fixed system is accurately in parallel to the moving axis of the carriage and that the beam is accurately deflected toward the direction perpendicular to the moving axis of the carriage by the optical element mounted in the carriage. Therefore, with regard to the separation type device, the accuracy of the optical element regarding the shape and size or the assembly location and attitude is more critically required in comparison to the one unit type device which is not divided to two optical systems to avoid the problems, mentioned above.
There is a method aiming at avoiding the misalignment of the optical axis of the objective lens and that of the beam incident to the lens due to the errors in the shape or size of the prism itself or the mounting position or attitude of the prism when assembling the device. The method is such that the laser source is arranged movable in the direction of the axis Z (vertical direction) and the direction of the axis Y (horizontal direction) with respect to the coupling lens.
However, the above-mentioned method also involves in the problem that the optical axis of the laser beam emitted from the source is inclined with respect to the optical axis of the coupling lens due to the errors in the shape and size of the prism, the beam splitter or the quarter-wave plate and those of the carriage.
Another example of the optical pickup device relating to the present invention comprises a carriage in which a focus control unit for moving the objective lens along its optical axis is mounted and a fixed optical system which includes a track control unit comprising a galvano-mirror for tracking, whereby aiming at reduction of the access time.
However, this structure requires further strict accuracy of the optical elements and the assembly thereof than the structure in which the tracking means is disposed in the carriage, mentioned before, since the optical beam is inclined to transmit it to the objective lens by rotating the mirror so as to carry out the tracking operation.